I probably should have warned you that this could be tough. You don't get to see this stage of these animals very often. Did you have any guesses?
Here's a slightly different view. I can tell you that there are five tentacles clustered at one end, and two tube feet at the other end. And if you are having trouble judging, this animal is very small: All of these pictures were taken under a microscope.
The fact that there are tube feet will help you get to echinoderm. And you might recognize that this doesn't look quite right for a sea star, sea urchin, or sand dollar.
This is a pentacula — the late larval stage of a sea cucumber. It was caught in a plankton tow off Bodega Head, but because it has two tube feet, it's probably close to the time when it's ready to leave the plankton and settle on the bottom.
In fact, this individual was spending a lot of time walking around on the bottom of the dish used in these photographs. You can see it holding on with its tube feet and tentacles (see below).
As I was playing around with the lighting, trying to get the best view, I noticed that the tentacles had an interesting texture at their tips:
I zoomed in for a closer look:
Can you see all of those small, rounded projections?
Eric took an even better picture under higher magnification (200x):
I had to do a little research, but eventually I found a paper that described these structures. And I must say that I'm guessing Eric has just shared with you one of the best photographs of larval sea cucumber tentacle papillae ever taken!
(Don't they look like little Stegosaurus plates?)
Here's a drawing from the paper by Chia and Buchanan in which they illustrate the papillae at the tips of the tentacles:
Modified from Chia, F.-S. and J.B. Buchanan. 1969. Larval development of Cucumaria elongata (Echinodermata: Holothuroidea). J. Mar. Biol. Ass. U.K. 49: 151-159.
Chia and Buchanan describe the papillae as outgrowths of mucus cells — and they say that the mucus is important in both feeding and locomotion.
[We've also labeled the coelomocytes as drawn because some of these small, rounded cells are visible in the photograph above! (Look carefully at the base of the right-hand tentacle.) Coelomocytes can act as scavenger cells — roaming around and engulfing foreign substances.]
Oh, did you say that you wished you could see this pentacula in action? Well, you're in luck — Eric captured a few moments and created this short video clip. Watch the pentacula swimming, extending its tentacles, and holding on with its tube feet:
It seems there's a lot less known about the development of sea cucumber larvae because they're very difficult to study in the lab. But now you've been introduced to a young larva and you can think about it swimming in the plankton and settling down to start life as a tiny, microscopic sea cucumber. It will develop more tube feet, and its tentacles will branch into the tree-like structures that you're more familiar with.
Many thanks to Emily for sharing this discovery with us!